What is Cohort? Indication and Elements of Cohort Study. What is Relative risk and Attributable risk, and its interpretation? Advantages & disadvantages of Cohort study. Difference between Case control & Cohort study.

1. COHORT STUDY
Dr. Animesh Gupta
M.B.B.S., M.D., F.D.M., F.A.G.E.
Associate Professor
Department of Community Medicine, NMCH, Jamuhar
(Bihar)

2. SLO
At the end of this class, the students should know
✓ What is Cohort?
✓ Indication and Elements of Cohort Study.
✓ What is Relative risk and Attributable risk, and its
interpretation?
✓ Advantages & disadvantages of Cohort study.
✓ Difference between Case control & Cohort study.
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3. Epidemiology
Defined by John M. Last in 1988
“Study of distribution and determinants of health
related state or event in a specified population
and the application of this study to the control of
health problem”.
Measurement –
Disease frequency
Diseases distribution
Determinants of disease.
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4. Epidemiological Methods
OBSERVATIONAL STUDIES
DESCRIPTIVE STUDY ANALYTICAL STUDIES
✓ TIME
✓ PLACE
✓ PERSON
➢ ECOLOGICAL STUDY
➢ CROSS SECTIONAL STUDY
➢ CASE-CONTROL STUDY
➢ COHORT STUDY
EXPEREMENTAL STUDIES
➢ RANDOMIZED CONTROLLED TRIAL (RCT)
➢ FIELD TRIAL
➢ COMMUNITY TRIAL 4/25/2020Dr. Animesh Gupta Cohort Study
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5. WHAT IS COHORT?
Ancient Roman Military
unit, A band of warriors.
Persons banded
together.
Its a group of people
who share a common
characteristic or
experience. [Latin]
E.g. age, birth date,
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6. Cohort Study
Also k/a
Longitudinal
Prospective studies
Forward looking study
Incidence study
Starts with people free of disease
Assesses exposure at “baseline”
Assesses disease status at “follow-up”
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7. INDICATION OF A COHORT STUDY
When there is good evidence of exposure and
disease.
When exposure is rare but incidence of disease
is higher among exposed
When follow-up is easy, cohort is stable
When ample funds are available
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8. General consideration while selection of
Cohorts
Both the cohorts are free of the disease.
Both the groups should equally susceptible to
disease
Both the groups should be comparable
Diagnostic and eligibility criteria for the disease
should be defined well in advance.
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9. Design
To examine a relation between a factor and the
disease:
All subjects are free from the disease at the
beginning
Subjects are categorised on the basis of
presence or absence of exposure to the risk
factor
Subjects are then followed over time to
determine who develops the disease
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10. Study Design
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People
without
the
disease
Exposed
Not
Exposed
Disease
No
Disease
Disease
No Disease

11. Elements of cohort study
Selection of study subjects
Obtaining data on exposure
Selection of comparison
group
Follow up
Analysis
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12. Selection of study subjects
General population
Whole population in an area
A representative sample
Special group of population
Select group
 occupation group / professional group (Dolls study )
Exposure groups
Person having exposure to some physical, chemical or
biological agent
e.g. X-ray exposure to radiologists
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13. Obtaining data on exposure
Personal interviews / mailed questionnaire
Reviews of records
Dose of drug, radiation, type of surgery etc
Medical examination or special test
Blood pressure, serum cholesterol
Environmental survey
✓ By obtaining the data of exposure we can classify
cohorts as
Exposed and non exposed and
By degree exposure we can sub classify cohorts
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14. Selection of comparison group
Internal comparison
Only one cohort involved in study
Sub classified and internal comparison done
External comparison
More than one cohort in the study for the
purpose of comparison
e.g. Cohort of radiologist compared with
ophthalmologists
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15. Selection of comparison group….
Comparison with general population rates
If no comparison group is available we can
compare the rates of study cohort with
general population.
Cancer rate of uranium miners with cancer in
general population
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16. Follow-up
To obtain data about outcome to be determined
(morbidity or death)
Mailed questionnaire, telephone calls, personal
interviews
Periodic medical examination
Reviewing records
Surveillance of death records
Follow up is the most critical part of the study
✓ Some loss to follow up is inevitable due to death change
of address, migration, change of occupation. 4/25/2020
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17. ANALYSIS
Calculation of incidence rates among exposed
and non exposed groups
Estimation of risk
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18. INCIDENCE RATES OF OUTCOME
a b a + b
c d c + d
a + c b + d
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EXPOSURE
STATUS
DISEASE STATUS
TOTAL
STUDY COHORT
COMPARISON COHORT
YES
NO
YES NO
❑ Incidence among exposed = a/ (a+b)
❑ Incidence among non exposed = c/ (c+d)
TOTAL

19. Relative risk
✓ It is the ratio of the incidence of disease in
exposed compared to incidence in non
exposed
RR= incidence among exposed
incidence among non exposed
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20. Interpretation
If RR=1.0, then there is no association between
the exposure and the disease
If RR>1.0, then there is positive association
between the agent and disease
If RR<1.0, then there is negative association,
which means a curative effect of the agent on
the risk of disease. Eg: immunisation lowers the
risk of disease.
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21. Attributable risk
“Risk difference”
It is the difference in the incidence rate among
exposed to that of non exposed.
AR=
incidence rate among exposed – incidence rate among non exposed
incidence rate among exposed
AR indicates to what extent the disease under study
can be attributed to the exposure
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22. Types of Cohort Study
Prospective cohort study
Retrospective (historical) cohort study
Combination of Retrospective and Prospective
cohort study.
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23. time
Exposure Study starts
Disease
occurrence
Prospective Cohort Study
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Exposure
Disease
occurrence
time
Study starts

24. Retrospective cohort studies
Exposure
time
Disease
occurrence Study starts
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25. ADVANTAGES
Can establish cause-effect relationship
Can find out multiple diseases related to single
exposure
Can find the incidence rate and risk
Useful when exposure is rare
Minimises selection and information bias
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26. DISADVANTAGES
It often requires a large sample
Its ineffective for rare diseases
Losses due to follow up
Takes a long time for completion
Expensive
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27. Case Control Vs Cohort study
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Type Retrospective Prospective
Study design From outcome to exposure From exposure to
outcome
Indication When disease is rare When exposure is rare
Temporal association Not proven Proven
Duration Shorter or quickly longer
Sample size Less More
Outcome estimation Only odds ratio, neither
incidence or prevalence
Incidence as well as RR
Logistic efforts Cheap Expensive
Bias Recall, Berksonian Loss to follow up, cross
over bias

28. THANK YOU 4/25/2020Dr. Animesh Gupta Cohort Study
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